1. Field of the Invention
The field of the present invention relates in general to plasma reactors and processes typically used for wafer processing or the like. More particularly, the field of the invention relates to a wafer processing plasma reactor in which the plasma is generated primarily by inductively coupled power.
2. Background
A variety of approaches are currently in use for deposition, etching, resist removal, and related processing for semiconductor wafers and other substrates. A typical process may involve the placement of photoresist, the etching of areas unprotected by the resist, and the removal of the residual resist. One method for etching and resist removal involves the immersion of the substrate in a chemical bath that attacks the substrate or the resist. This method is generally known as wet chemical etching or stripping. While wet etching is conceptually straightforward, it has significant disadvantages. For instance, wet etching requires the storage, handling, and disposal of toxic chemicals. In addition, wet chemical etch is isotropic, that is, it occurs in all directions relatively equally. This leads to undesired lateral etching under the photoresist, called undercutting, which limits the dimensional control that can be achieved with wet etch.
An alternative processing technology involves the use of a plasma for deposition, etching, resist removal, or the like. A plasma is a nearly electrically neutral ionized gas. It contains a substantial density of free electrons and positively charged ions. To remain ionized, a gas must constantly receive energy to offset the recombination of charged particles which occurs mostly on the walls of the reactor chamber. In conventional anisotropic plasma processing, the pressure must also be kept low to reduce the collision rate of the ions. Plasmas for etching are typically formed by applying a radio-frequency (RF) electric field to a gas held at low pressure in a vacuum chamber.
Anisotropic plasma etching in the fabrication of integrated circuit devices is desirable because it can much better control feature dimensions and sidewall profiles by processing under conditions far from thermodynamic equilibrium. This enables the production of integrated circuit features having sidewalls with a precisely defined location that extend substantially vertically from the edges of the masking layer. This is important in devices that have a feature size and spacing comparable to the death of the etch.
In FIG. 1 is shown a typical wafer processing plasma reactor used for etching. This reactor includes a metal wall 1 that encloses a plasma reactor chamber 2. Wall 1 is grounded and functions as one of the plasma electrodes. Gases are supplied to chamber 2 from a gas source 3 and are exhausted by an exhaust system 4 that actively pumps gases out of the reactor to maintain a low pressure suitable for a plasma process. An RF power supply 5 is connected to a second, powered electrode 6 that capacitively couples power into the chamber 2 to form a plasma. A wafer 7 is positioned on or near powered electrode 6 for processing. Wafers are transferred into and out of the reactor chamber 2 through a port such as slit valve 8 or the like.
RF power at 13.56 MHz is predominantly used in plasma reactors because this frequency is an ISM (Industry, Scientific, Medical) standard frequency for which government mandated radiation limits are less stringent than at non-ISM frequencies, particularly those within the communication bands. The substantially universal use of 13.56 Mhz is further encouraged by the large amount of equipment available at that frequency because of this ISM standard. Other ISM standard frequencies are at 27.12 and 40.68 MHz, which are first and second order harmonics of the 13.56 MHz ISM standard frequency.
This energy is applied to a gas in the reactor chamber to form a plasma. The plasma consists of two qualitatively different regions: a quasineutral, equipotential conductive plasma body 9 and a boundary layer 10 called the plasma sheath. The plasma body consists of substantially equal densities of negative and positive charged particles as well as radicals and stable neutral particles. RF power coupled into the reactor chamber couples energy into the free electrons, imparting sufficient energy to many of these electrons that ions can be produced through collisions of these electrons with gas molecules. In addition to this ionization, dissociation and excitation of molecules occur within the plasma body. In dissociation, a molecule, such as O2, breaks down into smaller fragments, such as atomic oxygen. In excitation, the molecule holds together, but absorbs energy and enters an excited electronic state. Control of the relative amounts of ionization, dissociation, and excitation depends upon a variety of factors, including the pressure and level of energy applied to the plasma.
A plasma comprises a plurality of mobile charge carriers and thus is a conductive medium.
Therefore, the interior of the plasma is at a roughly uniform electric potential. However, the plasma cannot exist for long in contact with a material object, and is separated from objects by a so called sheath. The plasma sheath is an electron deficient, poorly conductive region in which the electric field strength is large. The plasma sheath has a perpendicular electric field between the plasma body and any interface with a material object such as the reactor walls and the electrodes. The electric field at the interface with the wafers causes ions to accelerate perpendicularly into the surface of the wafers. This perpendicular bombardment makes anisotropic etching possible. Typically, a wafer 7 is positioned on or slightly above the powered electrode 6 where there is a strong xe2x80x9cselfxe2x80x9d bias which enhances ion impact energy and thus the etching process so commercially viable etch rates may be obtained.
However, many modern IC structures are sensitive to ion bombardment by high energy ( greater than 100 eV) ions such as in the conventional plasma etch apparatus of FIG. 1. Other etching processes require even lower ion energies to maintain selectivity. Because wafer damage decreases with decreasing ion energy and associated sheath voltage, it would be advantageous to operate at smaller discharge power levels and voltages. Unfortunately, for capacitively coupled power at 13.56 MHz, this reduction of voltage results in a proportionately lower rate of creation of reactive species and ions and therefore results in a slower etch rate for many processes, which significantly degrades process throughput.
Etch rates depend upon the ion current density at the wafer surface and the sheath voltage at the powered electrode (which determines the energy of the ions bombarding the surface for etching). With decreased sheath voltage the ion current density at the wafer must be increased to maintain a substantially constant etch rate. This requires that the plasma ion density near the wafer be increased. Unfortunately, in a conventional plasma etcher, both the sheath voltage of the powered electrode and the ion density near that electrode increase or decrease together and are monotonically increasing functions of the amplitude of the RF voltage applied to the powered electrode. Thus, if the sheath voltage is decreased by decreasing the voltage of the RF signal, the ion current density at the wafer also decreases thereby producing an even greater decrease in the etch rate than would be the case from a decrease in the sheath voltage alone. It is desirable, therefore, to independently control the sheath voltage and ion density at the wafer so that a soft etch process (i.e., an etch process with reduced sheath voltage at the wafer) can be implemented that has a commercially adequate etch rate.
Induction coils surrounding a reactor chamber may be used to inductively couple RF energy into a plasma to control the composition and density of the plasma for semiconductor processing. A separate powered electrode adjacent to a wafer is used to accelerate ions toward the wafer surface for etching. These two mechanisms may be used to control the ion density and energies in the plasma, thereby allowing high etch rates with low ion energies.
However, problems are encountered with known methods of inductively coupling energy into a plasma reactor. The source of the inductive energy, such as induction coils, may also act to capacitively couple current into the reactor which modulates the potential of the plasma sheath at the frequency of the RF source. Thus, the source of inductive energyxe2x80x94used to control ion densityxe2x80x94may also affect the ions energies such that ion density and energy are not in fact independent. This may in turn degrade the symmetry of the etch and may damage substrates sensitive to voltage fluctuations. It will also increase the time-averaged plasma potential and thereby increase the energies of ions bombarding the wafer and may adversely affect the selectivity of the etch.
Thus, what is needed for etching is a method for independently controlling ion density and ion energy at a wafer surface. What is also needed is a method of inductively coupling energy into a plasma to produce ions without substantially modulating the sheath potential.
Plasma reactors are also used in processes that are even more sensitive to ion energies and modulation of the sheath potential, such as resist removal. After resist is used in the etching process, it must be removed. The resist is typically an organic film such as a hydrocarbon polymer. Typical resist removal involves the production of large amounts of atomic oxygen through dissociation. When exposed to the atomic oxygen, the hydrocarbon resist film will burn away. This particular process of resist removal is therefore often referred to as ashing. Additives or other gases may be used to remove other films in a similar manner.
Whereas etching depends upon bombardment by ions with relatively high energies, resist removal and similar processes depend upon the chemical reaction of an activated species (such as dissociated oxygen atoms) with a film on the semiconductor wafer. Typically, RF or microwave power is coupled into a gas to form a plasma. The plasma acts as a source of dissociated atoms or reaction with the film on the wafer.
In contrast to etch processes, however, it is not desirable to expose the semiconductor wafers directly to the plasma. Direct contact with the plasma will cause a high flux of charged particles to the wafer and a plasma sheath to form adjacent to the wafer surface, which may cause sputtering by ions from the plasma. The semiconductor wafers may thus be damaged either due to electrostatic charging, ion bombardment, or exposure to ultraviolet (UV) light, thus lowering the yield of the semiconductor wafers.
One approach to decreasing destructive sputtering is to generate the plasma remotely and provide the dissociated atoms to the wafers through a path containing sharp turns. Ions in the plasma collide with the electrodes used to generate the plasma and the chamber wall, but are substantially prevented from reaching the semiconductor wafers by the sharp turns.
A similar approach is described in U.S. Pat. No. 5,217,560 (Kurano). A plasma reactor as described in Kurano is shown in FIG. 2. The reactor comprises an outer reactor tube 11 and an inner tube 12. Gases are supplied through gas pipe 14. Electrodes 15 and 16 create electric fields, E, which cause a plasma to be formed between the outer and inner tubes 11 and 12. Holes 13 allow reactive gases to reach the wafers 17, while ions tend to collide with the inner tube 12.
However, a variety of disadvantages are associated with conventional plasma reactors used for resist removal and the like. First, complex structures may be necessary to isolate the plasma from the wafers. Second, providing atomic radicals to a wafer through a complex path may lead to the non-uniform flux of the radicals to the wafer. This decreases the uniformity of resist removal. Third, capacitive coupling of energy into the plasma drives currents and causes ions to collide with greater energy with the chamber wall and any electrodes in the plasma chamber. This may reduce the lifetime of the reactor (particularly if certain gas additives are used which may be needed for removal of residual chemicals from the xe2x80x9cashxe2x80x9d of the resist). This problem is present even when the coupled energy is primarily inductive, since conventional induction coils also capacitively couple energy into the plasma and modulate the plasma potential thereby driving electric currents through the plasma to the nearest metal surfaces. This may cause discharges to barriers used to separate the plasma and the wafer and may limit the effectiveness of any such barriers. If small holes are used in the barrier so few ions can pass through, modulation of the plasma potential may induce hollow cathode discharge in the holes or between the grids. This may actually increase the charged particle current passing through the barrier. Finally, dissociated gases may recombine before reaching the remote wafers, reducing the efficiency of the plasma as a source of atoms.
Thus, what is needed for resist removal and similar processes is a method for producing abundant dissociated atoms without producing excessive ions and without unnecessarily driving RF currents or causing charge buildup at chamber walls, electrodes, or semiconductor wafers. What is also needed is a plasma reactor that is capable of generating a plasma in a chamber and providing neutral activated species to a semiconductor wafer without bombarding the wafer with ions and without necessitating complex structures that degrade wafer processing. Preferably, a plasma with a low or highly localized density of charged particles may be produced, and the majority of those charged particles may be filtered from the neutral activated species before reaching the wafer. What is also needed is a method of inductively coupling energy into a plasma without substantially modulating the potential of the plasma.
One aspect of the present invention provides a method for inductively coupling energy into a plasma without substantially capacitively coupling energy into the plasma. Another aspect of the present invention provides a split Faraday shield interposed between a plasma and induction coil surrounding the plasma.
It is an advantage of these and other aspects of the present invention that energy may be provided to a plasma without substantially modulating the sheath potential of the plasma and without substantially driving ion currents into a chamber wall or semiconductor wafer. The split Faraday shield allows inductive energy to be coupled into the plasma while substantially preventing the capacitive coupling of energy into the plasma from the induction coil, thereby avoiding modulating the plasma potential.
Yet another aspect of the present invention provides a plasma reactor with a split Faraday shield interposed between a plasma in a reactor and induction coil surrounding the reactor, and also provides a powered electrode in the reactor adjacent to which a wafer or the like may be placed for processing.
It is an advantage of this and other aspects of the present invention that the induction coil may be used to control ion density in the plasma while the powered electrode may be used to substantially independently control ion energies for etching or the like. It is another advantage of this aspect of the present invention that a soft etch may be implemented while commercially viable etch rates are maintained.
Yet another aspect of the present invention provides a method of dissociating molecules in a plasma without substantially capacitively modulating the plasma potential. This aspect of the present invention may be implemented by interposing a split Faraday shield between induction coils and a reactor chamber.
It is an advantage of this and other aspects of the present invention that abundant dissociated atoms may be produced without requiring excessive electron density and without unnecessarily driving currents of charged particles or causing charge buildup at chamber walls, electrodes or semiconductor wafers. It is yet another advantage of this aspect of the present invention that a simple chamber structure may be used for resist removal or the like without substantially sputtering or otherwise damaging semiconductor wafers.
Another aspect of the present invention provides a substrate that is electrically isolated from a ground potential for holding a wafer or the like for processing. It is an advantage of this aspect of the present invention that the amount of RF current drawn to the substrate and wafer is reduced.
Yet another aspect of the present invention provides an electric field for filtering charged particles from an activated species before introducing the activated species to a wafer or the like. Another aspect of the present invention provides one or more grids for filtering charged particles from an activated species. Another aspect provides for inducing an electric field between grids to enhance filtering. Yet another aspect of the present invention provides for blocking UV radiation produced in a plasma from reaching a wafer or the like. This may be implemented by aligning grids so there is no line of sight from the plasma to the wafer.
It is an advantage of these and other aspects of the present invention that wafers or the like may be processed with activated species with reduced exposure to charged particles and UV radiation.